In resistance welding, two electrodes, typically of copper and cylindrical in shape, having spherical ends, are employed to apply relatively large electrical currents through metal to be welded. This flow of electric current results in heating of the metal to virtually melting temperature. Pressure is applied to the heated area through the electrodes to effectively forge the softened metal together to form a weld. The resulting weld joint is typically generally circular in area.
In the setting of a high-production environment, it is normally desirable to make a weld of this type in as short of a time interval as possible. This is typically accomplished by increasing the welding current and decreasing the time that the current is applied to a workpiece to be welded. However, in the event that welding in a current is increased too much, a phenomenon known as expulsion occurs. During expulsion, localized hotspots develop, when the temperature of the metal exceeds its melting point due to the metal being heated too quickly. The pressure of the welding electrodes, combined with magnetic effects, caused by the welding current, causes molten metal to be expelled from the weld.
Small amounts of weld expulsion are not seriously detrimental to weld quality; however, if expulsion becomes too severe, the strength of the weld is reduced due to the fact that some of the metal normally employed to form the weld area has been expelled. In addition to diminishment of weld quality, excessive expulsion results in sparks of hot metal, thus creating a safety hazard.
Due to the nature of the metal being welded, the current required to heat the metal to proper temperature to create an ideal weld in a reasonably short time interval, without excessive expulsion, is not a constant value. For example, in the case of welding workpiece materials such as galvanized or coated steel, physical changes occur to the surface of the welding electrodes which contact the workpiece. Over the course of effecting hundreds of welds, the spherical end of the welding electrodes becomes flattened out, thereby reducing the current concentration applied to the weld area. In the case of welding galvanized steel, the zinc on the surface of the steel alloys with the copper of the welding electrodes to form a layer of relatively low conductivity metal, i.e. bronze, on the working surfaces of the electrodes. These, and other effects, work in combination to cause the optimum welding current, i.e. sufficient current to weld quickly without excessive expulsion, to trend upwardly in magnitude as the welding electrodes deteriorate with use.
In the past, a so-called "stepper" function has been included in the welding controller in order to compensate for the welding process drift described above. Typically, after a number of welds are performed, the stepper function causes the welding current applied to be increased by a predefined small increment. Then, after a further certain number of welds are made, the welding current is again stepped up by some small increment. The number of welds and the weld current increase per step is usually user-programmed so that, based on experience, the operator can program these numbers so that the welding is increased in what he believes will be the proper amount as the welding electrodes deteriorate with continued use.
The above-described user-programmable steppers are less than completely satisfactory because they do not follow all process variations. For example, when welding galvanized steel, the rate of electrode deterioration is heavily influenced by variations in the thickness of the zinc coating. Under typical production conditions, variations of greater than 2 to 1 in the rate of electrode deterioration are common. In addition, other variables, such as electrode cooling water temperature and variations in welding electrode force, result in further uncertainty as to the rate of welding electrode deterioration. As a result of these process uncertainties, the operator can at best set the weld stepper program to provide a "best compromise" weld. The operator is thus forced to rely on time-consuming and costly weld test procedures to determine whether the weld stepper program is properly set up.
As alluded to above, the magnitude of welding current required to produce an ideal weld is that which is slightly less than the amount that causes weld expulsion. If the ideal current is slightly exceeded and a small amount of expulsion occurs, the affect on weld quality is quite small. However, this ideal amount of welding current shifts upwardly as the welding electrodes deteriorate with use, as previously discussed.
Thus, there is a need in the art for a method of controlling welding current so as to maintain it at ideal levels in spite of the various process variables described above.